Mapping multi uvs textures to a character in blender? 3 separate textures for character
Using the blend file you sent :
In the UV/image editor, click image and in the menu, new image (or Alt+N)


Select the size of your image (512x512 should be enough but let's use 1024x1024), give it a name (like Full Texture) and click ok

You now have a beautiful black image. This is where we'll bake your texture.
Now go in properties panel, click on vertex data tab and click on rkuv in UV Maps. Make sure the camera icon is active on UVMap.

In 3D view, enter Edit mode with TAB, salect all with A and in the UV/Image editor, select the Full Texture", then go back in object mode (TAB in 3D view)

In properties panel, go in render tab, in Bake, bake mode click to open the menu and select "Textures"

Click the Bake button. Your Full texture is baking...

In UV/Image Editor, click Image, Save As image to save your new texture (if you don't, it'll be lost!!)

In properties panel, go to material tab, select ks material

Now go in Texture tab, click the button to change texture and select your brand new Full Texture

Back in material tab, delete all but ks materials.

Go in Vertex Data tab and delete "UVMap"

Now you have only 1 material with only 1 texture. If it doesn't show, go in material tab, enter edit mode, select all and click the "assign" button. If your texture doesn't show, go in texture tab and click the refresh button.

now save your file (change the name to keep the original with textures) and you're done !

I don't know much about sims 4 studio, but if you need an OBJ file, open the file menu, select export then wavefront (.obj). give it a name and you'll be able to import it from sims 4 studio and select the Full Texture file as texture. (the old OBJ won't work)